Field of the Invention
The present invention relates to an insulated synchronous rectification DC/DC converter.
Description of the Related Art
Various kinds of consumer electronics devices such as TVs, refrigerators, etc., each operate receiving commercial AC electric power from an external circuit. Also, electronic devices such as laptop computers, cellular phone terminals, and tablet PCs are each configured to operate using commercial AC electric power, and/or to be capable of charging a built-in battery using such commercial AC electric power. Such consumer electronics devices and electronic devices (which will collectively be referred to as “electronic devices” hereafter) each include a built-in power supply apparatus (AC/DC converter) that performs AC/DC conversion of commercial AC voltage. Alternatively, in some cases, such an AC/DC converter is built into an external power supply adapter (AC adapter) for such an electronic device.
FIG. 1 is a block diagram showing an AC/DC converter 100r investigated by the present inventor. The AC/DC converter 100r mainly includes a filter 102, a rectifier circuit 104, a smoothing capacitor 106, and a DC/DC converter 200r. 
The commercial AC voltage VAC is input to the filter 102 via a fuse and an input capacitor (not shown). The filter 102 removes noise included in the commercial AC voltage VAC. The rectifier circuit 104 is configured as a diode bridge circuit which performs full-wave rectification of the commercial AC voltage VAC. The output voltage of the rectifier circuit 104 is smoothed by the smoothing capacitor 106, thereby generating a converted DC voltage VIN.
An insulated DC/DC converter 200r receives the DC voltage VIN via an input terminal P1, steps down the DC voltage VIN thus received so as to generate an output voltage VOUT stabilized to a target value, and supplies the output voltage VOUT thus stabilized to a load (not shown) connected between an output terminal P2 and a ground terminal P3.
The DC/DC converter 200r includes a primary-side controller 202, a photocoupler 204, a shunt regulator 206, an output circuit 210, a secondary-side controller 300r, and other circuit components. The output circuit 210 includes a transformer T1, a diode D1, an output capacitor C1, a switching transistor M1, and a synchronous rectification transistor M2. The output circuit 210 has the same topology as those of typical synchronous rectification flyback converters, and accordingly description thereof will be omitted.
The switching transistor M1 connected to the primary winding W1 of the transformer T1 performs switching so as to step down the input voltage VIN, thereby generating the output voltage VOUT. With such an arrangement, the primary-side controller 202 adjusts the duty ratio of the switching of the switching transistor Ml.
The output voltage VOUT of the DC/DC converter 200r is divided by means of resistors R1 and R2. The cathode (K) terminal of the shunt regulator 206 is connected to a light-emitting element (light-emitting diode) on the input side of the photocoupler 204. The anode (A) terminal of the shunt regulator 206 is grounded. The divided voltage (voltage detection signal) VOUT_S is input to a reference (REF) terminal of the shunt regulator 206. The shunt regulator 206 includes an error amplifier that amplifies the difference between the voltage detection signal VOUT_S and a reference voltage VREF (not shown) so as to generate an error current IERR that corresponds to the difference, which is drawn (as a sink current) via the light-emitting element (light-emitting diode) on the input side of the photocoupler 204.
A feedback current IFB flows through a light-receiving element (phototransistor) on the output side of the photocoupler 204 according to the error current IERR that flows on the secondary side. The feedback current IFB is smoothed by means of a resistor and a capacitor, and is input to a feedback (FB) terminal of the primary-side controller 202. The primary-side controller 202 adjusts the duty ratio of the switching transistor M1 based on the voltage (feedback voltage) VFB at the FB terminal.
The secondary-side controller 300r switches on and off the synchronous rectification transistor M2 in synchronization with the switching of the switching transistor M1. The secondary-side controller 300r includes a synchronous rectification controller 304 and a driver 306. The synchronous rectification controller 304 generates a pulse signal S1 in synchronization with the switching of the switching transistor M1. For example, when the switching transistor M1 turns off, the synchronous rectification controller 304 sets the pulse signal S1 to a first state (e.g., high level) configured as an instruction to turn on the synchronous rectification transistor M2. When the secondary current IS that flows through the secondary winding W2 becomes substantially zero in an on period of the synchronous rectification transistor M2, the synchronous rectification controller 304 sets the pulse signal S1 to a second state (low level) configured as an instruction to turn off the synchronous rectification transistor M2.
The driver 306 switches on and off the synchronous rectification transistor M2 according to the pulse signal S1. The above is the overall configuration of the AC/DC converter 100r. 
As a result of investigating the secondary-side controller 300r, the present inventors have come to recognize the following problems.
In order to generate the pulse signal S1, in many cases, the secondary-side controller 300r performs time measurement. The time measurement result may be used for edge blanking, a timing control operation for tuning on or turning off the synchronous rectification transistor M2, or a control operation for controlling the upper limit or the lower limit of the on time or the off time. Such a time period (which will be referred to as the “control time” hereafter) is required to be set as appropriate according to the time constant of the circuit elements of the output circuit 210. Accordingly, in many cases, the secondary-side controller 300r is configured to have a terminal (which will be referred to as the “SET terminal” hereafter) that allows the control time to be set via an external circuit.
In many cases, the SET terminal is connected to an external resistor or an external capacitor. With a typical example, a timer circuit is configured as a combination of a capacitor, a current source that charges the capacitor, and a voltage comparator that compares the voltage across the capacitor with a threshold voltage. With a configuration in which the SET terminal is connected to a setting resistor RSET configured as an external resistor, the current value generated by the current source may be adjusted according to the setting resistor RSET. Alternatively, with such a configuration, the threshold voltage may be adjusted according to the setting resistor RSET. Also, as an another configuration, the SET terminal may be connected to an external capacitor.
With the secondary-side controller 300r having such a SET terminal, if a short circuit (short circuit to a power supply or otherwise to the ground) or otherwise an open circuit occurs in the SET terminal due to dust or a fault in the mounting of the SET terminal, such an arrangement is not capable of measuring the control time with high precision. This leads to an abnormal operation of the synchronous rectification transistor M2.